If the ONLY thing that is slow is Yahoo Mail, then try gmail...
If EVERYTHING is bogged down, you need to 'clean up' (or upgrade) your computer.

Run C-Cleaner, Malwarebytes, Super Anti-Spyware, Defraggler, and your Anti-Virus program. I do this once a month, and when cleaned, create a restore point. All free apps that work!

Good Luck